Clinical and biological characterization of 20 patients with TANGO2 deficiency indicates novel triggers of metabolic crises and no primary energetic defect

Mitochondrial mayhem: Disrupting conserved N-terminal motifs in TANGO2 impacts its localization and function

TANGO2 deficiency in humans leads to progressive neurological impairment, punctuated by life-threatening metabolic crises. In this issue, Lujan and colleagues demonstrate that TANGO2 localizes within the mitochondrial lumen and binds acyl-CoA species, potentially implicating it as a lipid trafficking protein.

TANGO-2: A Rare Genetic Condition With Severe Clinical Presentation of Encephalopathy, Rhabdomyolysis, and Cardiac Rhythm Disorders in 2 Children

Biallelic pathogenic or likely pathogenic variants in Transport and Golgi Organization 2 (TANGO-2) are associated with a spectrum of clinical features including encephalopathy, rhabdomyolysis, cardiac rhythm disorders, and neurologic regression. We are reporting on 2 unrelated children with biallelic TANGO-2 pathogenic variants. These variants were identified through a Next Generation Sequencing (NGS) panel of genes associated with hereditary rhabdomyolysis. Both children had a history of developmental delay, especially in their motor milestones. They also experienced episodic transient weakness with acute illness. One of the children's siblings had similar complaints and died at an early age. During their illness, both children developed extreme lethargy with very high CPK levels, lactic acidosis, rising trends of transaminases, and recurrent hypoglycemia. Both patients developed ventricular tachyarrhythmias, and the echocardiogram showed cardiomyopathy. Despite intensive symptomatic management, both patients died of cardiac failure because of fatal ventricular arrhythmia. Genetic testing revealed the presence of biallelic pathogenic variants TANGO-2. This rare genetic condition should be suspected in any patient with episodic recurrent weakness, rhabdomyolysis, abdominal pain, and cardiac arrhythmias, because of its diverse clinical presentation. However, early diagnosis is challenging because there are no specific biochemical markers for the disease. There is strong evidence that vitamin B supplementation can significantly reduce the number of metabolic crises in these children. Although this is not a targeted therapy, it can be a potentially life-saving treatment for these patients.

Cellular mechanisms of acute rhabdomyolysis in inherited metabolic diseases

Acute rhabdomyolysis (RM) constitutes a life-threatening emergency resulting from the (acute) breakdown of skeletal myofibers, characterized by a plasma creatine kinase (CK) level exceeding 1000 IU/L in response to a precipitating factor. Genetic predisposition, particularly inherited metabolic diseases, often underlie RM, contributing to recurrent episodes. Both sporadic and congenital forms of RM share common triggers. Considering the skeletal muscle's urgent need to rapidly adjust to environmental cues, sustaining sufficient energy levels and functional autophagy and mitophagy processes are vital for its preservation and response to stressors. Crucially, the composition of membrane lipids, along with lipid and calcium transport, and the availability of adenosine triphosphate (ATP), influence membrane biophysical properties, membrane curvature in skeletal muscle, calcium channel signaling regulation, and determine the characteristics of autophagic organelles. Consequently, a genetic defect involving ATP depletion, aberrant calcium release, abnormal lipid metabolism and/or lipid or calcium transport, and/or impaired anterograde trafficking may disrupt autophagy resulting in RM. The complex composition of lipid membranes also alters Toll-like receptor signaling and viral replication. In response, infections, recognized triggers of RM, stimulate increased levels of inflammatory cytokines, affecting skeletal muscle integrity, energy metabolism, and cellular trafficking, while elevated temperatures can reduce the activity of thermolabile enzymes. Overall, several mechanisms can account for RMs and may be associated in the same disease-causing RM.

Current global vitamin and cofactor prescribing practices for primary mitochondrial diseases: Results of a European reference network survey

Primary mitochondrial diseases (PMD) account for a group of approximately 400 different genetic disorders with diverse clinical presentations and pathomechanisms. Although each individual disorder is rare, collectively they represent one of the largest groups in the field of inherited metabolic disorders. The complexity of PMD results in a continued lack of therapeutic options, necessitating a predominantly symptomatic treatment approach for affected patients. While a subset of diseases responds exceptionally well to treatment with specific vitamins or cofactors, for most PMD systematic reviews were not able to show significant benefit. This is in discrepancy to their continued frequent use among specialists. To gain further insight into the current clinical practice of vitamin and cofactor supplementation among clinicians treating children and adults affected by PMD, we conducted a worldwide cross-sectional questionnaire study exploring the choice of substances and the specific diseases where they are applied. To our knowledge, this is the first global study exploring this topic and featuring a high response rate from paediatricians. The vast majority (95%, 106/112) of responding specialists recommended the use of vitamins and cofactors, either in an agnostic approach irrespective of the specific PMD or directed to the treatment of specific diseases or phenotypes. Our study highlights significant regional and specialty-specific differences in supplementation practices. We provide some preliminary insights into specialist-based opinions regarding the use of vitamins and cofactors in PMD and highlight the need for more rigorous clinical and preclinical investigations and/or clear consensus statements.

Clinical case report of intractable paroxysmal sympathetic hyperactivity in TANGO2 deficiency disorder

TANGO2 deficiency disorder (TDD) is a neurodegenerative disease characterized by a broad and variable spectrum of clinical manifestations, even among individuals sharing the same pathogenic variants. Here, we report a severely affected individual with TDD presenting with intractable paroxysmal sympathetic hyperactivity (PSH). While progressive brain atrophy has been observed in TDD, PSH has not been reported. Despite comprehensive workup for an acute trigger, no definite cause was identified, and pharmacological interventions were ineffective to treat PSH. Ultimately care was redirected to comfort measures. This article expands the clinical phenotype of patients with TDD, highlights the possibility of PSH in these patients, and the need for continued research for better treatments of TDD.

Impaired coenzyme A homeostasis in cardiac dysfunction and benefits of boosting coenzyme A production with vitamin B5 and its derivatives in the management of heart failure

Coenzyme A (CoA) is an essential cofactor required for over a hundred metabolic reactions in the human body. This cofactor is synthesized de novo in our cells from vitamin B5, also known as pantothenic acid, a water-soluble vitamin abundantly present in vegetables and animal-based foods. Neurodegenerative disorders, cancer, and infectious diseases have been linked to defects in de novo CoA biosynthesis or reduced levels of this coenzyme. There is now accumulating evidence that CoA limitation is a critical pathomechanism in cardiac dysfunction too. In the current review, we will summarize our current knowledge on CoA and heart failure, with emphasis on two primary cardiomyopathies, phosphopantothenoylcysteine synthetase and phosphopantothenoylcysteine decarboxylase deficiency disorders biochemically characterized by a decreased level of CoA in patients' samples. Hence, we will discuss the potential benefits of CoA restoration in these diseases and, more generally, in heart failure, by vitamin B5 and its derivatives pantethine and 4'-phosphopantetheine.

Folate as a potential treatment for lethal ventricular arrhythmias in TANGO2-deficiency disorder

TANGO2-deficiency disorder (TDD) is an autosomal-recessive genetic disease caused by biallelic loss-of-function variants in the TANGO2 gene. TDD-associated cardiac arrhythmias are recalcitrant to standard antiarrhythmic medications and constitute the leading cause of death. Disease modeling for TDD has been primarily carried out using human dermal fibroblast and, more recently, in Drosophila by multiple research groups. No human cardiomyocyte system has been reported, which greatly hinders the investigation and understanding of TDD-associated arrhythmias. Here, we established potentially novel patient-derived induced pluripotent stem cell differentiated cardiomyocyte (iPSC-CM) models that recapitulate key electrophysiological abnormalities in TDD. These electrophysiological abnormalities were rescued in iPSC-CMs with either adenoviral expression of WT-TANGO2 or correction of the pathogenic variant using CRISPR editing. Our natural history study in patients with TDD suggests that the intake of multivitamin/B complex greatly diminished the risk of cardiac crises in patients with TDD. In agreement with the clinical findings, we demonstrated that high-dose folate (vitamin B9) virtually abolishes arrhythmias in TDD iPSC-CMs and that folate's effect was blocked by the dihydrofolate reductase inhibitor methotrexate, supporting the need for intracellular folate to mediate antiarrhythmic effects. In summary, data from TDD iPSC-CM models together with clinical observations support the use of B vitamins to mitigate cardiac crises in patients with TDD, providing potentially life-saving treatment strategies during life-threatening events.

TANGO2 deficiency disease is predominantly caused by a lipid imbalance

TANGO2 deficiency disease (TDD) is a rare genetic disorder estimated to affect ∼8000 individuals worldwide. It causes neurodegeneration often accompanied by potentially lethal metabolic crises that are triggered by diet or illness. Recent work has demonstrated distinct lipid imbalances in multiple model systems either depleted for or devoid of the TANGO2 protein, including human cells, fruit flies and zebrafish. Importantly, vitamin B5 supplementation has been shown to rescue TANGO2 deficiency-associated defects in flies and human cells. The notion that vitamin B5 is needed for synthesis of the lipid precursor coenzyme A (CoA) corroborates the hypothesis that key aspects of TDD pathology may be caused by lipid imbalance. A natural history study of 73 individuals with TDD reported that either multivitamin or vitamin B complex supplementation prevented the metabolic crises, suggesting this as a potentially life-saving treatment. Although recently published work supports this notion, much remains unknown about TANGO2 function, the pathological mechanism of TDD and the possible downsides of sustained vitamin supplementation in children and young adults. In this Perspective, we discuss these recent findings and highlight areas for immediate scientific attention.

Orchestrating vesicular and nonvesicular membrane dynamics by intrinsically disordered proteins

Compartmentalization by membranes is a common feature of eukaryotic cells and serves to spatiotemporally confine biochemical reactions to control physiology. Membrane-bound organelles such as the endoplasmic reticulum (ER), the Golgi complex, endosomes and lysosomes, and the plasma membrane, continuously exchange material via vesicular carriers. In addition to vesicular trafficking entailing budding, fission, and fusion processes, organelles can form membrane contact sites (MCSs) that enable the nonvesicular exchange of lipids, ions, and metabolites, or the secretion of neurotransmitters via subsequent membrane fusion. Recent data suggest that biomolecule and information transfer via vesicular carriers and via MCSs share common organizational principles and are often mediated by proteins with intrinsically disordered regions (IDRs). Intrinsically disordered proteins (IDPs) can assemble via low-affinity, multivalent interactions to facilitate membrane tethering, deformation, fission, or fusion. Here, we review our current understanding of how IDPs drive the formation of multivalent protein assemblies and protein condensates to orchestrate vesicular and nonvesicular transport with a special focus on presynaptic neurotransmission. We further discuss how dysfunction of IDPs causes disease and outline perspectives for future research.

Management of acute metabolic crisis in TANGO2 deficiency: a case report

OBJECTIVES

TANGO2 deficiency is a rare inborn error of metabolism, with distinct clinical features. The clinical presentations of TANGO2 deficiency are developmental delay, speech difficulties, intellectual disability, non-life-threatening paroxysmal neurologic episodes (TANGO2 spells), acute metabolic crises, cardiac crises, seizures and hypothyroidism. Patients may die in acute metabolic crises. Here we report our experience in the management of an acute metabolic crisis in TANGO2 deficiency.

CASE PRESENTATION

A 9-year-old patient diagnosed with TANGO2 deficiency was admitted with fever, fatigue, unable to walk. In follow up, encephalopathy, rhabdomyolysis and arrhythmia were detected. Vitamin B-complex was started. Our patient's mental status and rhabdomyolysis improved dramatically, and cardiac crises ended without Torsades de pointes, ventricular tachycardia and/or fibrillation or myocardial dysfunction.

CONCLUSIONS

With this report, we aimed to show the effectiveness of vitamin B-complex in the management of acute metabolic crises.

Expanding the phenotypic spectrum of TRAPPC11-related muscular dystrophy: 25 Roma individuals carrying a founder variant

BACKGROUND

Limb-girdle muscular dystrophies (LGMD) are a heterogeneous group of genetically determined muscle disorders. TRAPPC11-related LGMD is an autosomal-recessive condition characterised by muscle weakness and intellectual disability.

METHODS

A clinical and histopathological characterisation of 25 Roma individuals with LGMD R18 caused by the homozygous TRAPPC11 c.1287+5G>A variant is reported. Functional effects of the variant on mitochondrial function were investigated.

RESULTS

The c.1287+5G>A variant leads to a phenotype characterised by early onset muscle weakness, movement disorder, intellectual disability and elevated serum creatine kinase, which is similar to other series. As novel clinical findings, we found that microcephaly is almost universal and that infections in the first years of life seem to act as triggers for a psychomotor regression and onset of seizures in several individuals with TRAPPC11 variants, who showed pseudometabolic crises triggered by infections. Our functional studies expanded the role of TRAPPC11 deficiency in mitochondrial function, as a decreased mitochondrial ATP production capacity and alterations in the mitochondrial network architecture were detected.

CONCLUSION

We provide a comprehensive phenotypic characterisation of the pathogenic variant TRAPPC11 c.1287+5G>A, which is founder in the Roma population. Our observations indicate that some typical features of golgipathies, such as microcephaly and clinical decompensation associated with infections, are prevalent in individuals with LGMD R18.

TANGO2 Deficiency Disorder: Two Cases of Developmental Delay Preceding Metabolic Crisis

BACKGROUND

TANGO2 deficiency disorder is a rare genetic disease caused by biallelic defects in TANGO2 gene.

METHODS

We report the clinical phenotype of two children with TANGO2 deficiency disorder.

RESULTS

Patient 1 is a female child presenting with developmental delay and microcephaly during the second year of life, who evolved with severe cognitive impairment, facial dysmorphisms, spastic paraparesis, and atonic seizures. At age 13 years, she was hospitalized due to an episode of rhabdomyolysis complicated with cardiac arrhythmia and hypothyroidism. Patient 2 is a female child with dysmorphic facial features, cleft palate, and developmental delay who was diagnosed with DiGeorge syndrome. At age three years, she presented with an acute episode of severe rhabdomyolysis in the context of human herpesvirus 6 infection. After the resolution of this acute episode, she maintained recurrent muscle weakness with axial hypotonia and progressive spasticity of the lower extremities. In both patients, diagnosis of TANGO2 deficiency disorder was only confirmed after an acute metabolic crisis.

CONCLUSIONS

A high index of suspicion for TANGO2 deficiency disorder is needed in patients with developmental delay or other neurological symptoms and episodic rhabdomyolysis.

Intrinsic and extrinsic regulation of rhabdomyolysis susceptibility by Tango2

Rhabdomyolysis is a clinical emergency characterized by severe muscle damage, resulting in the release of intracellular muscle components, which leads to myoglobinuria and, in severe cases, acute kidney failure. Rhabdomyolysis is caused by genetic factors linked to increased disease susceptibility in response to extrinsic triggers. Recessive mutations in TANGO2 result in episodic rhabdomyolysis, metabolic crises, encephalopathy and cardiac arrhythmia. The underlying mechanism contributing to disease onset in response to specific triggers remains unclear. To address these challenges, we created a zebrafish model of Tango2 deficiency. Here, we demonstrate that the loss of Tango2 in zebrafish results in growth defects, early lethality and increased susceptibility of skeletal muscle defects in response to extrinsic triggers, similar to TANGO2-deficient patients. Using lipidomics, we identified alterations in the glycerolipid pathway in tango2 mutants, which is critical for membrane stability and energy balance. Therefore, these studies provide insight into key disease processes in Tango2 deficiency and have increased our understanding of the impacts of specific defects on predisposition to environmental triggers in TANGO2-related disorders.

Early initiation of B-vitamin supplementation may reduce symptoms and explain intrafamilial variability: Insights from two sibling pairs from the TANGO2 natural history study

TANGO2-deficiency disorder (TDD) is an autosomal recessive condition arising from pathogenic biallelic variants in the TANGO2 gene. TDD is characterized by symptoms typically beginning in late infancy including delayed developmental milestones, cognitive impairment, dysarthria, expressive language deficits, and gait abnormalities. There is wide phenotypic variability where some are severely affected while others have mild symptoms. This variability has been documented even among sibling pairs who share the same genotype, but reasons for this variability have not been well understood. Emerging data suggest a potential link between B-complex or multivitamin supplementation and decreased metabolic crises in TDD. In this report, we describe two sibling pairs from unreladiagnosed with TDD with marked differences in symptoms. In both families, the older siblings suffered multiple metabolic crises and are clinically more affected than their younger siblings who have very mild to no symptoms; they are the least impaired among 70 other patients in our ongoing international natural history study. Unlike their older siblings, the two younger siblings started taking B-complex vitamins early between 9 and 16 months. This report delineates the mildest presentation of TDD in two families. These data may support a role for early diagnosis and initiation of vitamin supplementation to not only prevent metabolic crises but also improve neurologic outcomes in this life-threatening disorder.

Natural history of TANGO2 deficiency disorder: Baseline assessment of 73 patients

PURPOSE

TANGO2 deficiency disorder (TDD), an autosomal recessive disease first reported in 2016, is characterized by neurodevelopmental delay, seizures, intermittent ataxia, hypothyroidism, and life-threatening metabolic and cardiac crises. The purpose of this study was to define the natural history of TDD.

METHODS

Data were collected from an ongoing natural history study of patients with TDD enrolled between February 2019 and May 2022. Data were obtained through phone or video based parent interviews and medical record review.

RESULTS

Data were collected from 73 patients (59% male) from 57 unrelated families living in 16 different countries. The median age of participants at the time of data collection was 9.0 years (interquartile range = 5.3-15.9 years, range = fetal to 31.8 years). A total of 24 different TANGO2 alleles were observed. Patients showed normal development in early infancy, with progressive delay in developmental milestones thereafter. Symptoms included ataxia, dystonia, and speech difficulties, typically starting between the ages of 1 to 3 years. A total of 46/71 (65%) patients suffered metabolic crises, and of those, 30 (65%) developed cardiac crises. Metabolic crises were significantly decreased after the initiation of B-complex or multivitamin supplementation.

CONCLUSION

We provide the most comprehensive review of natural history of TDD and important observational data suggesting that B-complex or multivitamins may prevent metabolic crises.

Vitamin B5, a coenzyme A precursor, rescues TANGO2 deficiency disease-associated defects in Drosophila and human cells

Mutations in the Transport and Golgi Organization 2 (TANGO2) gene are associated with intellectual deficit, neurodevelopmental delay and regression. Individuals can also present with an acute metabolic crisis that includes rhabdomyolysis, cardiomyopathy, and cardiac arrhythmias, the latter of which are potentially lethal. While preventing metabolic crises has the potential to reduce mortality, no treatments currently exist for this condition. The function of TANGO2 remains unknown but is suspected to be involved in some aspect of lipid metabolism. Here, we describe a model of TANGO2-related disease in the fruit fly Drosophila melanogaster that recapitulates crucial disease traits. Pairing a new fly model with human cells, we examined the effects of vitamin B5, a coenzyme A (CoA) precursor, on alleviating the cellular and organismal defects associated with TANGO2 deficiency. We demonstrate that vitamin B5 specifically improves multiple defects associated with TANGO2 loss-of-function in Drosophila and rescues membrane trafficking defects in human cells. We also observed a partial rescue of one of the fly defects by vitamin B3, though to a lesser extent than vitamin B5. Our data suggest that a B complex supplement containing vitamin B5/pantothenate may have therapeutic benefits in individuals with TANGO2-deficiency disease. Possible mechanisms for the rescue are discussed that may include restoration of lipid homeostasis.

Limb-girdle myopathy and mild intellectual disability: the expanding spectrum of TANGO2-related disease

TANGO2-related disease is an autosomal recessive multisystem disease associated with developmental delay and infancy-onset recurrent metabolic crises with early mortality. Several studies have reported dysfunction in endoplasmic reticulum-to-Golgi traffic and mitochondrial homoeostasis as the underlying pathophysiology. We report a 40-year-old woman affected by limb-girdle weakness and mild intellectual disability caused by the recurrent deletion of exons 3-9 in homozygosity in the TANGO2 gene. Physical examination revealed hyperlordosis, waddling gait, calf pseudohypertrophy, and Aquilian tendon retractions. Laboratory investigations revealed elevation of serum biomarkers suggestive of mitochondrial dysfunction together with hypothyroidism. At the age of 24, the patient suffered a metabolic crisis with severe rhabdomyolysis and malignant cardiac arrhythmia. After recovery, no metabolic or arrhythmic crisis has recurred. Muscle histology two years later revealed increased endomysial fibrosis and other myopathic changes. Our findings illustrate the mildest end of the phenotypic spectrum of TANGO2-related disease and reveal further aspects related to chronic muscle damage in this disorder.

The TANGO2 disease and the therapeutic challenge of acute arrhythmia management: a case report

Background

TANGO2-related metabolic encephalopathy and arrhythmia are a rare, newly recognized, and likely under-diagnosed condition. First described in 2016, it is characterized by developmental delay and recurrent metabolic crisis. During these episodes, patients may present QTc prolongation and ventricular arrhythmias.

Case summary

A 13-year-old female, with developmental delay, presented with severe rhabdomyolysis and an initially normal electrocardiogram (ECG). Due to the worsening of rhabdomyolysis, QTc prolongation was identified (QTc 570 ms) and oral β-blocker therapy started. A non-sustained ventricular tachycardia developed, initially managed with magnesium and lidocaine. After a short period, an arrhythmic storm of polymorphic ventricular extrasystoles induced Torsade de Pointes (TdP) was triggered. A temporary percutaneous pacing lead was placed and esmolol infusion started. The electrical instability ran in parallel with the increasing severity of rhabdomyolysis and systolic ventricular function decline. Genetic testing identified a pathogenic variant in homozygosity in the TANGO2 gene. A stable sinus rhythm was achieved with metabolic and serum electrolytes optimization. ECG showed normalization of the QTc interval.

Discussion

The full TANGO2-related phenotype emerges over time and the prognosis is linked to the appearance of ECG abnormalities. QT interval prolongation can lead to life-threatening ventricular tachycardias. The arrhythmia mechanism seems to be secondary to metabolite build-up in cardiomyocytes, which can explain the cardiac phenotype during the crisis which subsides after their resolution. In these patients, avoiding bradycardia is fundamental, since long QT-related TdP seems to be triggered by bradycardia and short-long-short ventricular premature beats (VPB). During an acute metabolic crisis, the management of arrhythmias relies on metabolic control.

Transport and Golgi organization 2 deficiency with a prominent elevation of C14:1 during a metabolic crisis: A case report

Mutations in transport and Golgi organization 2 homolog (TANGO2) have recently been described as a cause of an autosomal recessive syndrome characterized by episodes of metabolic crisis associated with rhabdomyolysis, cardiac arrhythmias, and neurodegeneration. Herein, we report a case of a one-and-a-half-year-old Japanese girl, born to nonconsanguineous parents, who presented with metabolic crisis characterized by hypoglycemia with hypoketonemia, rhabdomyolysis, lactic acidosis, and prolonged corrected QT interval (QTc) at the age of 6 months. Acylcarnitine analysis during the episode of crisis showed prominent elevation of C14:1, suggesting very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. In addition, worsening rhabdomyolysis was observed after intravenous administration of L-carnitine. VLCAD deficiency was initially suspected; however, the enzyme activity in lymphocytes was only mildly decreased at the gene carrier level, and no mutation in the VLCAD gene (ADADVL) was detected. Subsequently, acylcarnitine analysis was nonspecific at 17-h fasting and almost normal during the stable phase. Eventually, a trio whole-exome sequencing revealed a compound heterozygous variant of two novel variants in the TANGO2 gene, a missense variant, and a deletion of exon 7. This is the first case of TANGO2 deficiency in Asians. Our case suggests that elevated C14:1 may be seen in severe metabolic crises and that the use of L-carnitine should be avoided during metabolic crises.

Cardiac crises: Cardiac arrhythmias and cardiomyopathy during TANGO2 deficiency related metabolic crises

BACKGROUND

TANGO2 deficiency disorder (TDD) is an autosomal recessive disease associated with metabolic crisis, lethal cardiac arrhythmias, and cardiomyopathy. Data regarding treatment, management, and outcomes of cardiac manifestations of TDD are lacking.

OBJECTIVE

The purpose of this study was to describe TDD-related cardiac crises.

METHODS

Retrospective multicenter chart review was made of TDD patients admitted with cardiac crises, defined as development of ventricular tachycardia (VT), cardiomyopathy, or cardiac arrest during metabolic crises.

RESULTS

Twenty-seven children were admitted for 43 cardiac crises (median age 6.4 years; interquartile range [IQR] 2.4-9.8 years) at 14 centers. During crisis, QTc prolongation occurred in all (median 547 ms; IQR 504-600 ms) and a type I Brugada pattern in 8 (26%). Arrhythmias included VT in 21 (78%), supraventricular tachycardia in 3 (11%), and heart block in 1 (4%). Nineteen patients (70%) developed cardiomyopathy, and 20 (74%) experienced a cardiac arrest. There were 10 deaths (37%), 6 related to arrhythmias. In 5 patients, recalcitrant VT occurred despite use of antiarrhythmic drugs. In 6 patients, arrhythmias were controlled after extracorporeal membrane oxygenation (ECMO) support; 5 of these patients survived. Among 10 patients who survived VT without ECMO, successful treatment included intravenous magnesium, isoproterenol, and atrial pacing in multiple cases and verapamil in 1 patient. Initiation of feeds seemed to decrease VT events.

CONCLUSION

TDD-related cardiac crises are associated with a high risk of arrhythmias, cardiomyopathy, cardiac arrest, and death. Although further studies are needed, early recognition and appropriate treatment are critical. Acutely, intravenous magnesium, isoproterenol, atrial pacing, and ECMO as a last resort seem to be the best current treatment options, and early initiation of feeds may prevent VT events.

Mitochondria-Associated Membrane Scaffolding with Endoplasmic Reticulum: A Dynamic Pathway of Developmental Disease

Communication between intracellular organelles is essential for overall cellular function. How this communication occurs and under what circumstances alterations transpire are only the beginning to be elucidated. The pathways of calcium homeostasis, lipid transfer, mitochondrial dynamics, and mitophagy/apoptosis have been linked to the endoplasmic reticulum and tethering sites on the outer and/or inner mitochondrial membrane called mitochondria-associated endoplasmic reticulum membranes (MAM). Sensitive visualization by high-powered microscopy coupled with the advent of massive parallel sequencing has elaborated the structure, while patient’s diseases have uncovered the physiological function of these networks. Using specific patient examples from our pediatric mitochondrial center, we expand how specific genetic pathological variants in certain MAM structures induce disease. Genetic variants in MICU1, PASC-2, CYP2U1, SERAC1, and TANGO2 can induce early development abnormalities in the areas of cognition, motor, and central nervous system structures across multiple MAM pathways and implicate mitochondrial dysregulation.

Anesthetic Challenges in a Patient With TANGO2 Gene Deletion, DiGeorge Syndrome, and Tetralogy of Fallot: A Case Report

Mutations of the transport and Golgi organization 2 (TANGO2) genes are linked with both long-term neurological decline and acute metabolic crises during stress, leading to significant anesthetic risk. Crises are marked by rhabdomyolysis, lactic acidosis, seizures, cardiac dysfunction, and dysrhythmias. Much is unknown about optimal management of this condition, especially in the acute and critical care settings. The following report describes the anesthetic challenges of a patient with simultaneous TANGO2 gene deletion, DiGeorge Syndrome, and Tetralogy of Fallot, who presented for an interventional cardiac procedure with the goal of metabolic crisis-avoidance and facilitation of safe but expeditious recovery and discharge home.

Sudden Cardiac Arrest in the Paediatric Population

Sudden cardiac arrest in the young is a rare event with a range of potential causes including cardiomyopathies, ion channelopathies, and autonomic nervous system dysfunction. Investigations into the cause involve a multidisciplinary team, including cardiologists, geneticists, and psychologists. In addition to a detailed medical history, family history and circumstances surrounding the event are important in determining the cause. Clinical investigations including an electrocardiogram are fundamental in diagnosis and should be interpreted cautiously because some children may have atypical presentations and an evolving phenotype. The potential for misdiagnosis exists that could lead to incorrect long-term management strategies. If an inherited condition is suspected, genetic testing of the patient and cascade screening of family members is recommended with genetic counselling and psychological support. Medical management is left to the treating physician acknowledging that a clear diagnosis cannot be made in approximately half of cases. Secondary prevention implantable defibrillators are widely deployed but can be associated with complications in young patients. A plan for safe return to activity is recommended along with a proper transition of care into adulthood. Broad screening of the general population for arrhythmia syndromes is not recommended; preventative measures include screening paediatric patients for risk factors by their primary care physician. Several milestone events or activities that take place in youth could be used as opportunities to promote safety. Further work into risk stratification of this paediatric population through patient registries and greater awareness of cardiopulmonary resuscitation and automated external defibrillator use in saving lives is warranted.

Mitochondrial dysfunction associated with TANGO2 deficiency

Transport and Golgi Organization protein 2 Homolog (TANGO2)-related disease is an autosomal recessive disorder caused by mutations in the TANGO2 gene. Symptoms typically manifest in early childhood and include developmental delay, stress-induced episodic rhabdomyolysis, and cardiac arrhythmias, along with severe metabolic crises including hypoglycemia, lactic acidosis, and hyperammonemia. Severity varies among and within families. Previous studies have reported contradictory evidence of mitochondrial dysfunction. Since the clinical symptoms and metabolic abnormalities are suggestive of a broad dysfunction of mitochondrial energy metabolism, we undertook a broad examination of mitochondrial bioenergetics in TANGO2 deficient patients utilizing skin fibroblasts derived from three patients exhibiting TANGO2-related disease. Functional studies revealed that TANGO2 protein was present in mitochondrial extracts of control cells but not patient cells. Superoxide production was increased in patient cells, while oxygen consumption rate, particularly under stress, along with relative ATP levels and β-oxidation of oleate were reduced. Our findings suggest that mitochondrial function should be assessed and monitored in all patients with TANGO2 mutation as targeted treatment of the energy dysfunction could improve outcome in this condition.

FDX2 and ISCU Gene Variations Lead to Rhabdomyolysis With Distinct Severity and Iron Regulation

Background and Objectives

To determine common clinical and biological traits in 2 individuals with variants in ISCU and FDX2, displaying severe and recurrent rhabdomyolyses and lactic acidosis.

Methods

We performed a clinical characterization of 2 distinct individuals with biallelic ISCU or FDX2 variants from 2 separate families and a biological characterization with muscle and cells from those patients.

Results

The individual with FDX2 variants was clinically more affected than the individual with ISCU variants. Affected FDX2 individual fibroblasts and myoblasts showed reduced oxygen consumption rates and mitochondrial complex I and PDHc activities, associated with high levels of blood FGF21. ISCU individual fibroblasts showed no oxidative phosphorylation deficiency and moderate increase of blood FGF21 levels relative to controls. The severity of the FDX2 individual was not due to dysfunctional autophagy. Iron was excessively accumulated in ISCU-deficient skeletal muscle, which was accompanied by a downregulation of IRP1 and mitoferrin2 genes and an upregulation of frataxin (FXN) gene expression. This excessive iron accumulation was absent from FDX2 affected muscle and could not be correlated with variable gene expression in muscle cells.

Discussion

We conclude that FDX2 and ISCU variants result in a similar muscle phenotype, that differ in severity and skeletal muscle iron accumulation. ISCU and FDX2 are not involved in mitochondrial iron influx contrary to frataxin.

Variable clinical severity in TANGO2 deficiency: Case series and literature review

Biallelic pathogenic variants in the TANGO2 (transport and Golgi organization 2 homolog) gene have been identified as causing a rare metabolic disorder characterized by susceptibility to recurrent rhabdomyolysis, lactic acidosis, encephalopathy, and life-threatening tachyarrhythmias. Recently published reports suggest variable clinical severity and phenotypes. This study details five new patients from two families with biallelic pathogenic variants in the TANGO2 gene identified by whole exome sequencing and includes the largest number of affected individuals from a single family reported to date. We document significant intrafamilial variability and highlight that milder phenotypes may be underrecognized. We present biochemical and clinical data to help highlight the features that aid in consideration of this condition in the differential with disorders of fatty acid oxidation. We also present a comprehensive literature review summarizing the molecular, clinical, and biochemical findings for 92 individuals across 13 publications. Of the 27 pathogenic variants reported to date, the recurrent exons 3-9 deletion represents the most common variant seen in 42% of individuals with TANGO2 deficiency. Common clinical features seen in >70% of all individuals include acute metabolic crisis, rhabdomyolysis, neurologic abnormalities, developmental delay, and intellectual disability. Findings such as elevated creatine kinase, hypothyroidism, ketotic hypoglycemia, QT prolongation, or abnormalities of long-chain acylcarnitines and urine dicarboxylic acids should raise clinical suspicion for this life-threatening condition.

Symptom Prevalence and Genotype-Phenotype Correlations in Patients With TANGO2-Related Metabolic Encephalopathy and Arrhythmias (TRMEA)

BACKGROUND

TANGO2-related metabolic encephalopathy and arrhythmias (TRMEA) is a rare, phenotypically heterogeneous, neurological disease affecting children.

METHODS

We conducted a chart review of five children with molecularly confirmed TRMEA diagnosed at our institution and compiled pathogenic variant frequency and symptom prevalence from cases previously reported in the literature.

RESULTS

Including those patients in our case series, 76 patients with TRMEA have been described. Developmental delay (93%) and/or regression (71%), spasticity (78%), and seizures (57%) are common in TRMEA and frequently precede life-threatening symptoms such as metabolic decompensation with lactic acidosis (83%), cardiomyopathy (38%), and cardiac arrhythmias (68%). Deletion of exons 3 to 9 is the most common pathogenic variant (39% of alleles). The majority of reported intragenic variants (17 of 27) result in disruption of the reading frame, and no clear genotype-phenotype correlations could be identified for those variants wherein the reading frame is maintained, highlighting instead the variable expressivity of the disease.

CONCLUSIONS

Patients with TRMEA frequently experience life-threatening complications that are preceded by common neurological symptoms underscoring the need for pediatric neurologists to be familiar with this condition. Additional work pertaining to disease pathophysiology and potential therapeutics is needed.